Mould in London Properties: Condensation Versus Structural Damp — How to Tell the Difference

Why Getting the Diagnosis Right Matters

Mould is frequently misdiagnosed in London properties, leading to wasted money on treatments that do not address the root cause and ongoing disputes between landlords and tenants about responsibility. The three main causes — condensation, penetrating damp, and rising damp — look similar on the surface but have entirely different solutions and entirely different liability implications.

Condensation: The Lifestyle Cause

Condensation occurs when warm, moisture-laden air meets a cold surface. In London's typical Victorian and Edwardian terraces with solid brick walls, the inside wall surface temperature drops significantly in cold weather, particularly in unheated rooms or at cold bridges around window frames and external corners.

Diagnostic indicators of condensation:

• Mould appears first on cold surfaces: external corners, north-facing walls, around window reveals, behind wardrobes placed against external walls.
• Condensation on windows (streaming water) is present.
• Mould is typically black (Cladosporium or Aspergillus/Penicillium species), superficial, and can be wiped off with a mould spray.
• Symptoms are worst in winter and improve in summer without any structural intervention.
• No tide marks or plaster damage — the wall plaster is dry to the touch when the mould is cleaned off.

Causes: Insufficient ventilation, inadequate heating, drying laundry indoors without extraction, cooking without extractor fans, and high occupancy relative to property size all increase moisture load in the air.

Penetrating Damp: The Structural Cause

Penetrating damp is water ingress through the building fabric — failed pointing, cracked render, defective gutters, faulty window seals, or roof defects. It is a structural problem and the landlord's responsibility to fix.

Diagnostic indicators of penetrating damp:

• Damp patches appear only during or after rain, or worsen in wet weather.
• Patches are often irregular in shape and correspond to external defects — typically below a parapet, near a leaking gutter, or around a window frame.
• The plaster behind the mould is wet or soft, not just surface-level.
• Tide marks and salt efflorescence (white crystalline deposits) may be visible as the damp dries and rewets.
• Patches appear on upper floors or at high level on walls, inconsistent with rising damp.

Rising Damp: Less Common Than Marketed

Rising damp is groundwater drawn up through porous masonry by capillary action where the damp-proof course (DPC) is absent, bridged, or failed. It is genuine but significantly less common than the damp-proofing industry implies — many "rising damp" diagnoses are actually condensation or penetrating damp.

Diagnostic indicators of genuine rising damp:

• A distinct tide mark at a consistent height (usually no more than 1–1.2 metres above floor level) across a continuous external wall.
• Salt efflorescence at the tide mark.
• Damage to skirting boards and plaster at low level that does not improve in summer.
• The damp is confined to ground-floor level on external or party walls.
• A damp meter reading confirms moisture in the masonry at low level that diminishes higher up.

Landlord and Tenant Duties

Structural causes (penetrating damp, rising damp): These are the landlord's responsibility under Section 11 of the Landlord and Tenant Act 1985. The landlord must repair the defect causing moisture ingress and remediate any resulting mould or plaster damage.

Condensation: Responsibility is shared. The landlord must provide adequate heating, ventilation (extractor fans in bathrooms and kitchens), and insulation. The tenant must use the heating, ventilate the property, and not create excessive moisture loads. The Homes (Fitness for Human Habitation) Act 2018 and the Housing Health and Safety Rating System (HHSRS) both address cases where condensation arises from structural deficiencies rather than tenant behaviour.

Since the Awaab's Law provisions under the Social Housing (Regulation) Act 2023 and the extension of similar principles to the private rented sector under the Renters (Reform) Act 2024, landlords face tighter timeframes for addressing mould complaints. A landlord who ignores a documented mould complaint in a private tenancy faces potential enforcement action and Rent Repayment Orders.

What to Do First

1. Photograph and document the mould extent and location.
2. Note the pattern: does it worsen in rain? In winter only? In specific rooms?
3. Check externally for obvious defects: overflowing gutters, cracked pointing, failed render.
4. If the cause is unclear, an independent damp survey from a member of the Property Care Association (PCA) — not a firm that also sells damp treatments — gives an unbiased diagnosis.
5. Report in writing to the landlord (if a tenant), or commission structural repairs (if an owner-occupier) before applying mould treatments, which only address the symptom.